US 7757544 B2
The preferred embodiments are directed to a method and apparatus of operating a scanning probe microscope (SPM) including oscillating a probe of the SPM at a torsional resonance of the probe, and generally simultaneously measuring an electrical property, e.g., a current, capacitance, impedance, etc., between a probe of the SPM and a sample at a separation controlled by the torsional resonance mode. Preferably, the measuring step is performed while using torsional resonance feedback to maintain a set-point of SPM operation.
1. An apparatus for measuring a sample comprising:
a probe holder supporting a probe having a tip extending substantially orthogonally to the sample;
a torsional resonance mode actuator that couples energy to said probe to oscillate said probe at a torsional resonance thereof;
a conductor, wherein said conductor is in electrical communication with said probe;
an electrical property measuring detector for measuring an electrical property of the sample, wherein the sample is disposed between said probe and said conductor; and
a feedback loop that operates to maintain a probe-sample separation less than about 10 nm based on changes in the torsional oscillation of the probe.
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This application is a non-provisional application claiming priority to U.S. patent application Ser. No. 11/133,802 filed on May 21, 2005, now U.S. Pat. No. 7,155,964, which is a provisional conversion of provisional application No. 60/573,464 filed May 21, 2004, and is a continuation-in-part of U.S. patent application Ser. No. 10/189,108 filed on Jul. 2, 2002, now U.S. Pat. No. 6,945,099, and Ser. No. 10/937,597 filed on Sep. 9, 2004 now U.S. Pat. No. 7,168,301, the entirety of each of which is expressly incorporated by reference herein.
1. Field of the Invention
The present invention is directed to operating a probe-based instrument in torsional resonance mode, and more particularly, a method and apparatus of performing electrical property measurements using torsional resonance feedback.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “scanning probe microscopy.”
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant by maintaining constant deflection of the cantilever. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography.
Alternatively, some AFMs can at least selectively operate in an oscillation “flexural mode” of operation in which the cantilever oscillates generally about a fixed end. One popular flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Independent of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers typically fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
In the field of nanoscience and nanotechnology, it is extremely important to measure the electrical properties of various kinds of samples on the nanometer scale. Several techniques have been developed for this task. Among these techniques, scanning tunneling microscopy (STM), conductive AFM and scanning capacitance microscopy (SCM) are widely used. In STM, the sharp metal probe described above is brought close to a surface to be scanned, with a bias voltage applied between the tip and the surface. As known from quantum mechanics, there is some finite probability that electrons will tunnel through the insulating gap between the tip and the sample when the potential between the two is different, and the separation is small. This tunneling current is measured, and a feedback system changes the tip-surface distance to maintain a constant current at a set point as the tip is scanned across the sample. STM can be used to measure properties of metals, semiconductor and other materials with high to medium conductivities.
Notably, STM has a significant drawback. Since it uses the tunneling current as the feedback signal, the sample area being scanned needs to have some conductivity to allow the feedback loop to work throughout the scan. In general, STM cannot be used to scan an insulating sample or a conductive with insulating surface layers such as oxide. To overcome this problem, one known atomic force microscope, described in U.S. Pat. No. 5,874,734, uses a conductive probe with a sharp tip on a lever arm which is brought into contact with the surface of a sample to be scanned. The force of contact between the tip and the sample is measured by the deflection of the lever arm, with the feedback system moving the tip, or alternatively, the sample, up and down to maintain a constant force between the two during relative scanning motion produced by the AFM. During scanning, a constant or variable bias voltage may be applied between the tip and sample and the current distribution may be measured, preferably simultaneously.
The advantage of this technique is using the deflection force between probe and surface as the feedback signal to control the tip surface distance and force. The technique works on insulating samples with conductive patches and ultra-low conductivity samples. However, one drawback of this technique is that it uses the previously described contact mode of AFM operation, according to which a static vertical deflection force is utilized as the feedback signal to control the force, and thus the tip-surface distance, during scanning. There are several problems associated with this. First, the feedback can only maintain a constant force between the tip and the surface in the vertical direction. When the tip scans across the surface, there is generally a large shear force present, and this high lateral force can easily damage both the tip and the sample. Moreover, to run the microscope under stable imaging conditions, the tip and sample surface must remain in mechanical contact. This is a problem not only because of the high shear forces present in contact mode, but also because the measurement is useless if the tip and sample are not in contact. This issue has seriously limited the use of AFM-based electrical characterization techniques on soft samples like conductive polymers.
Next, the sensitivity of contact mode is limited because feedback is based on a static signal, as opposed to a dynamic signal. Static signals are more susceptible to thermal drift and charging, and thus sensitivity is compromised, as described in more detail below. For these reasons, contact mode is also not preferred when imaging soft and delicate samples.
These problems with contact mode operation led to the development of the previously described oscillating mode of AFM operation, see, e.g., U.S. Pat. No. 5,519,212. Again, in oscillating mode, a cantilever with a tip is driven to resonance at its flexural resonance frequency. The amplitude of the cantilever's flexural oscillations (between 20 nm and 100 nm) and the deflection angle of the cantilever (<100 nm) are detected by a quadrant photodetector, which outputs a voltage proportional to these two values. As the tip approaches a sample surface, the flexural oscillation (tapping) amplitude starts to decrease due to confinement of the surfaces of the tip and the sample. The flexural oscillation amplitude decreases to zero as the cantilever is lowered to the surface and therefore pushes the tip against the sample surface with an increasing contact pressure. Variation of amplitude between zero (contact) and free oscillation is used to control tip surface distance and force. Properties of the sample surface such a topography, hardness and electromagnetic properties can be acquired by raster scanning the tip over the sample surface, or vice versa, and controlling the tip/surface distance using the detected flexural oscillation amplitude.
Notably, in this regard, this oscillating mode feedback used to control tip-sample separation comprises dynamic signals. This is in contrast to contact mode which employs static signal feedback that reflects the absolute value of the acquired signal indicative of the motion of the cantilever at a certain point. With no reference, static signal feedback is susceptible to thermal drift and electrostatic charging, creating significant problems given that these phenomena directly affect the sensitivity of the measurement, as understood in the art. Active signals, on the other hand, reflect a relative shift in the acquired signals. By considering relative changes, dynamic signals are less affected by thermal drift and electrostatic charging. As a result, techniques that operate based on active signals are generally much more sensitive than those that rely on static signals, providing a significant advantage to oscillating mode.
Oscillating mode, as a dynamic measurement, benefits from a high “Q” value of the corresponding cantilever in air. The Q factor of a resonating cantilever is the width of the frequency response of the cantilever at half its maximum amplitude, divided by the resonance frequency of the cantilever. Notably, a higher Q factor in cantilever oscillation improves the signal-to-noise ratio in measurements that rely on variations in the amplitude and phase of cantilevers. The Q factor also reduces the effective force applied by the tip to the sample. As a result, TappingMode imaging is typically performed at much lower forces than contact mode, allowing routine imaging of much softer samples. Finally, in oscillating mode, the tip-surface contact time is a small fraction of the oscillation cycle, so the interaction force is mainly vertical and the shear force is dramatically reduced.
However, in oscillating mode, the cantilever resonant frequencies are generally greater than 10 kHz. Amplifiers sensitive enough to measure currents in the 60 fA range, which is of particular interest in the present case, are limited to bandwidths well below 1 kHz. Further, in oscillating mode as noted previously, the tip is in contact with the surface for a small fraction (for example 1%) of the oscillation cycle. Although this is a benefit for minimizing shear force, and thus sample damage, such minimal contact is a drawback in that the tip moves in and out of what is known as the “near field.” Because the tip of the probe must be in the near field to measure many electrical properties of the sample this often is a significant drawback with using oscillating mode to perform electrical measurements. Overall, for these reasons, it is generally not possible to perform low current measurements on samples while operating in oscillation feedback mode.
Therefore, one is most often left with operating the AFM in contact mode to perform these types of electrical measurements. As noted previously, in contact mode, there always must be mechanical contact between the tip and the sample surface, and thus a minimal force must always be applied. In fact, the tip must be caused to penetrate a water layer that resides on the sample surface and then to push into the sample surface; otherwise, stable imaging cannot be achieved. Therefore, a “soft” cantilever, i.e., one having a low spring constant, is typically employed in contact mode so as to minimize the magnitude of the forces applied to the sample surface, and thus minimize damage to the sample. In some cases, stiffer cantilevers are used to break through surface oxides and contaminants to make sound electrical contact.
Contact mode cantilevers often suffer from a serious drawback in that they cannot be brought into close proximity to the sample surface without the tip snapping into contact with the surface. This is due to the water layer on the sample surface that produces a meniscus force that acts on the lever as the tip approaches the sample surface. This is particularly a problem when performing force spectroscopy measurements, an illustration of which is discussed in connection with
When performing a spectroscopy or force measurement, probe-sample separation is controlled at a single scan location, i.e., X-Y location, as the deflection of the cantilever is monitored with an optical deflection detection scheme, for example. Typically, the tip is brought into contact with the sample surface at a certain speed and then withdrawn from the surface. As shown in
This outcome is illustrated graphically in
The field of electrical property measurement was therefore in need of a system that enables nanometer scale measurement of ultra-low currents, for example (60 fA to 120 pA), correlated with topography on soft and delicate materials, and to acquire STM-type tunneling current data without relying on current as the feedback mechanism. The ideal solution would reduce tip wear and increase throughput for measurements of thickness and electrical properties of dielectric and insulating films. Preferably, the system would enable stable and localized measurements of I/V curves both in contact with the surface and with a small (i.e., nanometer scale) vertical offset from the surface. A system that allows the probe tip to remain in the near field (preferably, within a couple of nanometers of tip-sample separation), yet ensures that the probe does not snap into contact with the sample surface, would be ideal.
The preferred embodiments overcome the above-noted drawbacks by providing an atomic force microscopy apparatus and method that characterizes electrical properties of samples that consist of both conductive and non-conductive regions on the nanometer scale. Local electrical properties that can be measured by this technique include, but are not limited to: voltage, current, conductivity, resistivity, conduction current, tunneling current, dI/dV, dI/dz, surface potential, work function, capacitance, dC/dV, dC/dz, dielectric constant, dopant density, impedance, barrier height, and induced photovoltage. All of these are examples of localized electrical measurements that can be made by SPM. The system and method provide direct correlation of the location on a sample surface with its electrical properties by simultaneously mapping the topography and electrical current obtained when a bias is applied between the tip and the sample. In particular, the preferred embodiments enable nanometer scale measurement using ultra-low currents (60 fA to 120 pA) on very soft and delicate materials. In the preferred embodiments, TR mode is employed whereby the amplitude of the torsional resonance of the probe is used to control the probe so that it remains in close proximity to the sample surface, thus allowing precise measurement of the current between the tip and surface, at narrow (i.e., nanometer scale) tip-sample separations, during data acquisition. Alternatively, the probe may be oscillated at a lateral resonance.
According to a first aspect of the preferred embodiment, a method of operating a scanning probe microscope (SPM) includes the steps of oscillating a probe of the SPM at a resonance of the probe, the resonance being at least one of a torsional resonance and a lateral resonance, and measuring an electrical current between a probe of the SPM and a sample. Preferably, in addition, the measuring step is performed while using torsional/lateral resonance feedback to maintain a set-point of SPM operation.
In another aspect of this embodiment, the sample is one of a group including a conducting polymer, an organic LED, a biomolecule, a carbon nanotube, a nanowire, a semiconductor, and a biological cell.
In a further aspect of this embodiment, the set-point is indicative of a separation between a tip of the probe and a surface of the sample. Preferably, the separation is less than 10 nm. More preferably, the separation is between about 1 and 5 nm.
According to another aspect of this embodiment, the electrical property is an electrical current, and the measuring step includes using a pA-amplifier to generate a current output signal.
In a still further aspect of this embodiment, the probe is conductive and a DC bias voltage is applied between the probe and the sample.
According to yet another aspect of this embodiment, the probe is sufficiently stiff to maintain a tip-sample separation of between about 1 nm and 5 nm during operation. Preferably, the probe has a spring constant between about 1 and 40 N/m.
According to a further aspect of this embodiment, the probe is a self-actuated probe having a cantilever including an active element. Preferably, the self-actuated probe includes a piezoelectric drive actuator disposed on a backside of a cantilever of the self-actuated probe.
According to another aspect of this embodiment, the method further includes applying a DC bias voltage between the tip of the probe and a surface of the sample so as to generate a current between the tip and the surface. Preferably, the current is less than 500 pA. More preferably, the current is between about 60 fA and 120 pA.
According to a still further aspect of this embodiment, the electrical property is a capacitance and is indicative of an SCM measurement. Notably, the method also preferably includes simultaneously measuring topography of the sample.
In another embodiment, a method of operating a scanning probe microscope includes measuring an electrical property of a sample using a probe, and maintaining a force between the probe and the sample at an amount generally less than 50 nN. Preferably, the force is maintained at less than 25 nN, and ideally is kept to about 1 nN, sometimes even in the attractive force region (2˜3 nN).
In a still further embodiment, a method of operating a scanning probe microscope (SPM) includes measuring an electrical property of a sample with a probe. Notably, the sample can have an elastic modulus of generally less than 1 GPa. Even so, the measuring step is performed without plastic deformation of the sample.
In another preferred embodiment, a method of performing an electrical property measurement using a scanning probe microscope (SPM) having at least one cantilever includes oscillating the cantilever at or near at least one of a torsional resonance and a lateral resonance. In addition, the method includes measuring a property of the oscillating cantilever, and adjusting the probe-sample separation as a result of the measuring the property step. In this case, the method measures a current between the sample and the probe, wherein a tip of the probe is separated from a surface of the sample by about 0 nm to 5 nm.
In yet another aspect of the preferred embodiment, an apparatus for measuring a sample includes a probe holder supporting a probe. In addition, the apparatus includes a torsional resonance mode actuator that couples energy to the probe to oscillate the probe at a torsional resonance thereof, and a conductor in electrical communication with said probe. Finally, an electrical property detector is provided to measure an electrical property of the sample when disposed between said probe and said conductor. Preferably, the probe holder is shielded.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.
A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:
The preferred embodiments take advantage of the benefits provided by a new mode of AFM operation being developed known as torsional resonance mode (“TR mode), shown and described for instance in U.S. Ser. Nos. 10/189,108 and 10/937,597 to the present assignee. This technique includes driving a cantilever probe at its torsional resonance frequency and using the amplitude of oscillation at resonance for feedback control to keep the tip in proximity to the sample surface. As with other modes of AFM operation, these feedback signals are indicative of a characteristic of a sample, such as sample topography, and can be used to generate a map of the surface. In TR mode, the torsional resonance of the lever and changes thereto are measured by a four-quadrant photo detector. More particularly, the detector differentiates both vertical and horizontal components of probe motion, indicative of, cantilever flexural and torsional motion, respectively.
The damping of the RMS amplitude of the detected torsional signal is used as the measure of tip surface interaction when the probe is brought into the proximity of the sample surface. A feedback loop is used to adjust the tip-sample separation based on a measurement of the torsional oscillation. For example, the feedback loop can adjust the tip-sample separation in response to the amplitude, phase, frequency or other measure of the torsional resonance. The feedback loop can move either the tip, the sample or both to adjust the tip-sample separation. Ideally the feedback loop is optimized to keep the tip-sample separation substantially constant, although some variation in this separation inevitably occurs. The feedback loop may be analog, digital, or a hybrid of both. The feedback loop may use a simple PI (proportional/integral) algorithm, or may use any type of more complex control schemes, including model-based controllers H2 or H-infinity control. Control theory textbooks contain many such candidate control algorithms.
In contrast to flexural oscillating mode (Tapping Mode), however, the tip is kept in close proximity to the surface for the entire oscillation (duty) cycle. This makes it possible to apply a bias between the tip and the sample and measure the resulting current passing through the tip while simultaneously mapping the topography of the surface. Such measurements could previously only be done, with significant drawbacks, in contact mode, in which the large associated shear forces typically damage delicate samples, thus making obtaining reasonable data of such samples basically impossible. Moreover, by using TR mode, rather than contact mode, a cantilever having a significantly greater stiffness (i.e., spring constant) can be used, and thus the probe can be positioned to measure electrical parameters accurately in the near field without the snap-to-contact problem associated with “soft” cantilevers used in contact mode, as described above in the Background.
In TR Mode imaging, the cantilever can be positioned such that it is being pulled downwardly by the attractive forces in the vertical direction between the tip and sample. In contrast to the repulsive force contact mode, the small attractive forces in TR Mode can typically be optimized to not damage the tip or sample. Also, TR Mode is an oscillating mode of operation, and as such is a dynamic measurement. Therefore, TR Mode benefits from a high “Q” value of the corresponding cantilever in air. Again, a higher Q factor in cantilever oscillation improves the signal-to-noise ratio in measurements that rely on variations in the amplitude and phase of the cantilever of the probe. As a result, for this additional reason, the sensitivity of measurements when operating in TR Mode is higher than when operating in contact mode.
Because of the high Q oscillation and the reduced normal force, the lateral forces between the tip and sample are much smaller than the static shear forces. In this case, the energy dissipation in TR Mode can be the surface dilation (i.e., elastic deformation of the surface). Therefore, TR Mode can image soft and delicate samples, similar to TappingMode, but keep the tip in the near field of the surface all the time. As such, it is possible to use torsional mode to keep the tip at a constant height at a particular sample area, then ramp the bias between the tip and sample to obtain local force spectroscopy information, similar to both contact current-voltage curves and the current-voltage curves that are obtained in scanning tunneling microscopy, often referred to as scanning tunneling spectroscopy (STS).
Another problem resolved by the preferred embodiments relates to one of the drawbacks of STM described previously. In STM, the entire sample surface must be conductive since the topographic feedback relies on the tunneling current between the tip and the sample in order for the tip to remain on the surface. By using the torsional resonance amplitude for topographic feedback, the tip can be maintained at a position slightly above the surface (fractions of a nanometer to nanometer range), out of mechanical contact with the sample, and acquire STM-type tunneling data without relying on keeping a continuous current in order to maintain the topographic feedback loop. Of course, as mentioned immediately above, this is possible also because a cantilever having a greater stiffness than a typical contact mode cantilever is employed, i.e., small tip-surface separations can be maintained without the tip snapping into contact with the sample surface.
Deflection of the probe 36 is monitored by a detector 38, which typically is an optical detection scheme (e.g., a laser beam-bounce system using a quadrant photo-detector). Detector 38, however, can be any device for providing a measurement of the motion of the cantilever. Other examples of suitable detectors include but are not limited to optical interferometry, capacitive detection, piezoelectric, and piezoresistive detection.
The output of detector 38 is transmitted to a signal processor 40 which may be used to obtain a measure of the torsional oscillation. In the simplest implementation, the signal processor consists of hardware and/or software to measure the amplitude of torsional oscillation. Alternatively or additionally the hardware and/or software may be used to measure the phase, frequency or other parameter of the torsional oscillation. Note that this signal processor may be entirely an analog circuit, for example an RMS-to-DC converter, a lock-in amplifier, and/or a phase-locked loop. Alternately, the high frequency cantilever deflection signal can be directly sampled by a high-speed data acquisition system and the acquired signal can be demodulated using digital computation either with dedicated digital circuitry, digital signal processors, FPGAs and/or computers. In yet another alternative, a hybrid system combining any of the analog and digital components mentioned above can be used to achieve the desired signal conditioning.
Signal processor 40 operates to condition the acquired signal for comparison to a desired set-point. The set-point may be set by a user or determined automatically by the system. The set-point is typically subtracted from the output of the signal processor at error node 42. The subtraction at error node 42 can be done by analog circuitry, for example with an instrumentation amplifier or operational amplifier. Alternatively, the set-point subtraction can be done by digital electronics or inside a computer. Error node 42 outputs an error signal that is sent to a controller 44. Controller 44 outputs a signal that is used to control the motion of one or more z-actuators to adjust the tip-sample separation. Controller 44 may be implemented by analog circuitry, digital electronics (for example, a digital signal processor, a field programmable gate array, computer), or a hybrid of any of the above. The controller 44 loop may use a simple PI (proportional/integral) feedback algorithm, or may use any type of more complex control schemes, including model-based controllers H2 or H-infinity control, as mentioned above.
Controller 44 generates a control signal that generally returns the torsional resonance oscillation to the selected set-point. More particularly, the control signal is applied to an actuator 46, for moving either the probe and/or sample vertically. Note that in this case the term “vertically” is used for convenience, referring to the primary axis of separation between the probe and the sample. There is no requirement, however, that this direction be oriented in the conventional up-down direction. Actuator 46 is preferably a piezoelectric actuator that expands and contracts when a voltage is applied to it. The piezoelectric actuator may be additionally guided or constrained by flexures or other means. In addition, the actuator may be an individual element or part of a multi-axis scanner, for example an XYZ piezoelectric tube. The z-actuator may also be a balanced momentum actuator, as disclosed in U.S. Pat. Nos. 6,323,483 and 6,861,649, for example. In alternative embodiments the actuator can be an electrostrictive, electrostatic, electromagnetic, magnetic, or other devices that produce motion in response to an input signal.
Finally, instrument 30 includes an electrical property detector 34 which preferably is a low-noise pA-amplifier 48 for measuring currents between the probe and the sample in the picoamp range. Electrical property detector 34 preferably also includes an extra gain stage and a filter stage (described below) that ultimately outputs signals to an ADC (analog-to-digital converter) to enable current measurements in the desired range of about 60 fA. The electrical property detector may also be a device for measuring such properties as voltage, current, conductivity, resistivity, conduction current, tunneling current, dI/dV, dI/dz, surface potential, work function, capacitance, dielectric constant, impedance, and induced photovoltage. For example, a device for resistivity measurements applies a voltage across the tip/sample gap and measures the resulting current. For a surface potential measurement, an additional feedback loop adjusts a voltage on the tip until it substantially matches the potential on the surface. If the desired electrical property is impedance, a vector impedance meter, manufactured for example by Agilent, would be the electrical property detector 34. Additionally, this device can be an “applications module” an add-on unit for example sold by the assignee which enables specific electrical measurements. For example, the present assignee sells applications modules for scanning spreading resistance, scanning capacitance microscopy, tunneling AFM (TUNA), and four-point probe measurements. The electrical property detector 34 is shown schematically as a single box, and it may consist of just a simple electronic circuit like an amplifier. Alternately, depending on the desired measurement, it may also consist of more sophisticated electronics that may implemented in part or in full in a computer or other digital electronics. For all the electrical property measurements described above, there are numerous publications in the literature describing the details of such measurements and thus they will not be described in more detail here. The key requirement, however, is that the electrical property detector 34 measures one or more electrical properties of the sample and/or the tip-sample junction.
A review of the preferred apparatus associated with each of these blocks 32 and 34 is described immediately below. Initially, AFM operation in torsional resonance mode is discussed with respect to
A probe-based instrument, such as an AFM, that analyzes one or more characteristics of a sample in a torsional resonance mode is shown in
Note that when the term “horizontal motion” is used herein, it is intended to refer to the motion that is produced by the torque that rotates the tip. However, because the scale of the arc that is traced out by the motion of the tip, motion is effectively only in the horizontal direction in torsional resonance mode. This motion is typically on the order of 0.3 nm to 3 nm. Also, in contrast to TappingMode in which the tip contacts the surface approximately 1% of the time during data acquisition, in TR mode the tip is kept near the surface nearly 100% of the time.
A preferred embodiment of a torsional resonance mode atomic force microscope 50 is shown in further detail in
In this embodiment, probe assembly 70 is supported by actuator 56. Actuator 56 preferably defines an XYZ scanner 58 that may be implemented with, for example, a piezoelectric tube or piezoelectric stack that translates a free end 60, and thus probe assembly 70 coupled thereto, in three orthogonal directions. More particularly, the XY section of scanner 58 translates the probe along the sample surface 80, while the Z drive section of scanner 58 translates the probe assembly orthogonally to the sample surface, for example, in response to feedback from controller 44 during data acquisition. In alternate embodiments, the XYZ scanner may be replaced by separate elements, for example XY scanning of the sample and Z actuation of the probe or any permutation that generates relative motion between the probe and the sample.
To operate AFM 50 in torsional resonance mode, at least the tip 76, and preferably the entire cantilever motion of the probe 71 is initially driven into oscillation at or near a torsional resonance of probe 71 using any of the techniques described in U.S. Ser. Nos. 10/189,108 and 10/937,597, noted above. The separation between tip 76 and sample 78 is then reduced (e.g., by exciting actuator 58 in Z) to cause the two to interact. A beam of light “L” generated by probe oscillation detection system 54 (e.g., a laser 51), is directed towards a back 75 of cantilever 74 so that it is reflected therefrom. The reflected beam is then sensed by a detector 62. Preferably, detector 62 is a quadrant (i.e., four-cell) photodetector.
The interaction between tip 76 and sample 78 causes the reflected beam to translate laterally across detector 62. This lateral movement of the reflected light beam is indicative of one or more sample surface characteristics. More particularly, the oscillating motion of cantilever 74 is detected by the lateral cells of quadrant deflector 62 which produces a corresponding AC signal. Changes in this AC signal are indicative of surface and possibly other characteristics of the sample under study.
To quantify these characteristics, the AC signal output by detector 62 (i.e., lateral deflection signal) is transmitted to by analog and/or digital means to signal processing block 40. Signal processing block 40 outputs a signal associated with the lateral or torsional motion of the cantilever. Signal in this case refers to an analog signal, for example a voltage and/or a digital signal or data representative of the torsional motion of the cantilever. In this case, the lateral signal output by processing block 40 is then transmitted to a error node 42 that generates an error signal based on a predetermined set-point corresponding to the desired torsional oscillation. In this context, the error signal can be an analog voltage, a digital signal, and/or data inside a computer. The desired torsional oscillation may be defined in terms of the oscillation amplitude. Alternatively, the phase of the signal output by detector 62 can be analyzed and compared with an appropriate set-point phase of torsional oscillation. In this way, the phase or frequency signal can be used as the error signal of the feedback loop to control probe-sample interactions, also known as torsional resonance feedback. Rather than an RMS-to-DC converter, a lock-in amplifier or any other structure capable of performing the identified functions may be employed.
The error signal output by error node 42 is then transmitted to a controller 68 that compensates for the error by generating a control signal that returns the torsional oscillation of the probe back to the set-point value. Controller 68 may be a proportional-integral (P-I) gain stage in the feedback loop that generates and transmits a correction signal to XYZ scanner 58. In response to the correction signal, scanner 58 translates probe assembly 70 orthogonally to the surface of sample 78 (i.e., in “Z”) to return the oscillation of the probe 71 to the set-point value. Notably, it is typically the control signal output by controller 68 that provides the data pertaining to the sample surface.
In addition, when relative scanning movement between probe assembly 70 and sample 78 is employed, a map of sample surface 80 may be generated by plotting the correction signals generated by the controller 68 for each scan position. Again, the X-Y components of scanner 58 are used to position tip 76 at different locations of surface 80 of sample 78 to allow generation of the map. Alternatively, torsional mode can be employed to conduct a force measurement and generate a corresponding curve. In this measurement, the tip 76 interacts with the sample 78 at different “Z” positions, and the corresponding forces are measured. One notable experiment includes using the tip 76 to “pull” on a molecule on a surface of a sample. Such force measurements (e.g., measuring local stiffness) are described in U.S. Pat. No. 5,224,376, assigned to the present assignee, and expressly incorporated herein by reference.
Next, the separation distance between the tip 76 and the sample 78 is reduced so the two begin to interact. As a result of this interaction, the torsional motion of the probe 71 changes. These changes are sensed in Block 96 as the detection system (54 in
In Block 104, the control signal is transmitted to the XYZ scanner 58 of
Turning next to
Note that in the above discussion of torsional oscillation and feedback, the same device and method can oscillate the cantilever at a lateral resonant frequency instead of or in addition to a torsional resonance. The torsional resonance is preferred because it typically generates less motion at about the tip than lateral resonance, thus allowing higher resolution images. Choosing a lateral resonance instead of a torsional resonance is done by simply selecting the correct frequency for a given cantilever geometry, using standard vibration modal analysis and/or harmonic analysis techniques.
Next, as shown in
In operation, a DC bias voltage +/−12V, typically, is applied between probe 122 and sample 127 via a source 126. A measurement current flows between the conductive tip 124 and, for example, a thin dielectric film 128 that resides on the surface of sample 127. During operation, current is measured by pA-amplifier 48 and processed by gain/filter stage 130 prior to being transmitted to an analog-to-digital converter 132 where the current data can thereafter be collected and/or displayed for the user. Gain/filter stage 130 preferably provides a gain of at least 1 pA/V, with noise less than about 30 fA RMS. Notably, unlike amplitude oscillation mode, the resonant frequencies of operation in torsional resonance mode are smaller, thus facilitating ready current detection with pA-amplifier 48, as suggested above.
In one preferred embodiment, the apparatus shown in
While it scans the surface of the sample, probe 150 responds to the output of feedback loop 142 to ultimately map the topography of the surface of the sample. To operate at maximum scanning rate, the gain of second feedback loop 144 which controls Z-position actuator 146, is reduced to zero or some small value. As a result, at a scanning rate greater than about 500 microns/sec, the topography of the sample surface appears as the feedback control signal applied to self-actuating probe 150 by first feedback loop 142. In this case, Z position actuator 146 may be controlled, for instance, in a pre-programmed manner to follow the slope of sample surface or to eliminate coupling due to the lateral scanning of tip 154. For a detailed description of active probe technology, see, e.g., U.S. Pat. Nos. 6,189,374 and 6,530,266, owned by the present assignee.
In this embodiment, for quick Z response, the position of the torsional oscillating cantilever is controlled by a fast Z actuator 151 of self actuating probe 150. Fast Z actuator 151 may comprise a zinc oxide layer, for instance, and is operable to move the sensing cantilever up and down at high bandwidth. As understood in the art, piezoelectric Z tube 146 can change the tip-sample separation (in this case by moving probe 150) a larger distance than actuator 151, but at lower bandwidth. Torsional response is driven by the same actuator of the active probe 150. By making the probe conductive, a DC bias voltage can be applied between the tip and the sample surface so that current can be measured as described previously.
The desired torsional motion of the probe 122 is shown in further detail in
In operation, the detected lateral signal (e.g., sensed via quadrant photodetector 160) is conditioned by signal processor 40 (see, for instance,
Another electrical parameter associated with samples that can be measured with a probe-based instrument is capacitance. In this embodiment, the advantages of torsional resonance mode are taken advantage in scanning capacitance microscopy (SCM) to characterize selected electrical properties of samples.
In typical SCM, a nano-scale conducting tip is scanned across a sample surface, and a capacitance detector measures variations in the probe-sample capacitance C. Although having a variety of applications, one of the most common uses for SCM is semiconductor characterization, including dopant profiling, device characterization, and surface defect characterization. SCM can show carrier concentration profiles in two dimensions in existing semiconductor devices as well as the relationship of these profiles to critical device structures. The capacitance variation or gradient (dC/dV) provides a measure of the local carrier concentration density and carrier type (n or p), and thus can be used for high-resolution two-dimensional carrier (i.e., dopant) profiling. SCM is particularly useful for this task given that it provides spatial resolution in a range of about 10-20 nm.
More specifically, a metalized probe forms a metal-insulator-semiconductor (MIS) capacitor with the semiconductor sample. An AC bias applied between a scanning contact AFM tip and the sample generates capacitance variations, preferably measured using a gigahertz resonant capacitance sensor, providing sensitivity to variations in the attofarad range. Notably, the measured capacitance most often requires consideration of three capacitive contributions in series, the capacitance of the air in the vicinity of tip, the capacitance of an oxide layer on the sample, and the capacitance of the sample in the vicinity of tip and therefore must be deconvolved.
In addition to imaging, SCM can be used to produce dC/dV versus V curves, i.e., to illustrate the gradient as a function of bias voltage across the lateral scan path on the sample. In this case, typically, a DC sample bias can be ramped between two user-selected values while dC/dV (i.e., sensor output) is monitored and plotted. When used in semiconductor applications, dopant types are revealed by the sign of the signal while dopant levels are indicated by intensity of the signal.
Overall, conventional SCM has advantages in these applications, but given that SCM is normally operated in contact mode, it is often not particularly useful when the user wishes to image electrical properties of more delicate samples and with very high resolution (e.g., nanometer). In the present embodiment, a TR-SCM system is provided so that, rather than operating in contact mode, the probe is oscillated in the lower force torsional resonance mode. TR-SCM thus provides improved ability to electrically characterize delicate samples by insuring that tip-sample forces are maintained at less than 50 nN, for example. In doing so, SCM measurements are realized on the nano-scale.
To effectuate SCM, an AC sample bias voltage generated by a capacitance measurement block 220 is applied to chuck (i.e., a conductor) 212 (see inset, also showing an insulator-sample (e.g., silicon) interface). Block 220 is in communication with an electronics block 222 of the SPM head and each is in communication with the SPM controller and computer 224. Block 220 is also in communication with a UHF resonant capacitance sensor 226 to provide power to the sensor, tune the sensor and otherwise communicate data to system controller/computer 224. Preferably, as shown in
Generally, UHF electrical resonant capacitance sensor 226 provides the basis of this detection. The electrical resonator is connected to a conductive SPM probe via a transmission line, as shown. When the resonating probe tip is put in contact with a semiconductor, for instance, the sensor, transmission line, probe and sample all become part of the resonator. As a result, tip-sample capacitance variations will load the end, to the transmission line and change the resonant frequency of the system. As known, small changes in resonant frequency create enormous changes in the amplitude of resonance as measured in volts. This system has been shown to be sensitive to variations as small as attofarads (10−18 farads).
In operation, SCM induces the desired capacitance variations in the sample near the tip by applying an electric field between the scanning contact AFM tip and the sample. Preferably, this is accomplished using a kilohertz AC bias voltage applied to the semiconductor. The free carriers beneath the tip are alternately attracted and repulsed by the tip due to the alternating electric field. The alternating depletion and accumulation of charges under the tip may be modeled as a moving capacitor plate. The scanning capacitance microscope measures the movement of charges including carriers in semiconductor samples, which translates into a stronger signal for low carrier concentration and/or with samples having a layer of thin oxide.
Several images obtained using the preferred embodiments are shown in
Finally, TR-SCM images are shown in
Notably, an additional advantage of the preferred embodiments over known electrical characterization techniques such as STM is that tunneling current, as that term is understood in the art, is not required to perform the electrical measurements due to the fact that current feedback is not used. In this regard, three kinds of emissions between the probe and sample can possibly be observed including tunneling, Shottky emission and field emission. As a result, the term “current measurement”, and others like it, are used herein to refer to detection of any such current, not simply tunneling current.
The preferred embodiments are useful in a variety of applications and for electrical characterization of samples that were heretofore difficult to characterize. For instance, organic conductors and conducting polymers, which make up a litany of emerging devices, as well as soft polymers, can now readily be characterized. In addition, DNA deposited on conductors, gold films and carbon nanotubes, as well as nanowires made of nitrites, silicon, germanium, etc., which classically could not be electrically characterized with standard contact mode techniques can now be readily measured. This is primarily due to the fact that the preferred embodiments, unlike techniques that utilize contact mode and its relatively shear forces, can measure samples either loosely bound to a substrate or easily broken (e.g., DNA). Again, both current distribution over an entire sample surface, or a portion thereof, as well as spectroscopic force measurements (even on the nanometer scale) can be readily accomplished over a wide range of samples.
Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the present invention is not limited thereto. For example, the sample may be subjected to alternate environments (e.g., varied media, atmospheric conditions, etc.), and the corresponding torsional mode response analyzed. Notably, such alterations may change oscillation properties of the probe as desired by the user. It will be manifest that various additions, modifications and rearrangements of the features of the present invention may be made without deviating from the spirit and scope of the underlying inventive concept.